An artificial dental prosthesis such as a crown covers portions of a tooth surface and is normally fabricated away from the patient's mouth, in a lab, and then installed in the mouth by the dentist.
The artificial crowns are prepared based on a working cast (also known by the term “master cast”). It is on the cast that all the technical steps leading to the completed restoration must be performed. In preparing artificial crowns, best mechanical compatibility between the abutment tooth (hereinafter referred to as the preparation) and the crown is desired, to ensure complete imperviousness of the restored structure. Thus, the more precisely the working cast reproduces the anatomy of the mouth in the areas to be treated, the more accurate will be the spatial position as will be the static and dynamic relationships within the mouth after treatment. An accurate working cast is thus important to produce a biomechanically acceptable restoration.
The precision of the cast depends on several factors, including, inter alia, the accuracy of the impressions and wax bites, the material from which the cast is constructed, and the identification of the anatomic contours and of the finish line (also referred to at times by the term “chamfer line” and “marginal line”), etc.
The finish line, by definition, is the apical limit of the abutment tooth model (the “preparation”) and the margin of the reconstruction must end on it, i.e. it represents the point of transition between the biologic and artificial parts.
Being able to identify the zone that is apical to the finish line in absolute precision is fundamentally important for two reasons: (1) it allows to define the preparation limit with certainty, and (2) being intact, it maintains the anatomic characteristics of that tooth.
According to current practice, after diagnosing that a patient needs a crown, the dentist cuts the tooth to be reconstructed and prepares two impressions and a wax bite of the patient's jaws. Based on the impressions, wax bite and on written instructions of the dentist, a technician prepares in a lab the corresponding cast, and the relevant tooth within the preparation is temporary separated from the plaster so that the area with the anatomic information (the area defining the anatomic contour) and the finish line are exposed. At this point, the finish line is manually marked by the lab technician in ink on the preparation, and this finish line is an important parameter used in constructing the crown. Alternatively, a virtual three-dimensional (3D) image of the working cast is obtained e.g. in a manner as described in international publication No. WO97/03622, or in international publication No. WO00/08415, and the lab technician marks the finish line in the three dimensional environment.
U.S. Pat. No. 5,417,572 discloses a computer-based method for extracting a finish line for designing an artificial crown. Amounts of variation of data representing the shape of an abutment tooth are determined, and a train of points is extracted from the amounts of variation. Then a developed view of the surface shape of the abutment tooth is displayed, and the obtained train of points is also displayed in the developed view. The finish line for designing the artificial crown is determined, based on thus displayed train of points.
There are times when the finish line is not clear and the transition between the cut area to the biological area is not well defined. In such cases the technician either estimate himself where the line is or returns the cast (or the 3D virtual model) to the dentist for him to complete the finish line. In other cases, the boundaries between the cut area and the natural area of the tooth are blurred such that only the dentist himself is able to assess the cut area (the so-called ‘knife edges’), and to define the finish line.